Then their were two Thompson (not Thomas) Allens.Proof that Robt. Allen and Lydia Burch Allen had a son Thompson is abundant in Gilmer and Towns Co.Records.Thompson is shown as a minor after parents death in court records.Also, in grandfather Jarrett Burch's house in Union Co. Ga. also in Jarrett's will.I think it likely the Habersham Allens were related.
The Nancy Allen in 1840 Gilmer appears (not absolutely proven) to be the mother of Robt and Riley.Guessing her husband was named William, but there's support for that assumption.
Look for postings by Barbara Shedd as she has spent much time checking the early Habersham Allens.
What evidence does your book give to support the Habersham lineage?I am not really an Allen researcher, but have accumulated some data because many Allens appear to be associated with my Pinsons and associated families.